The use of railroad track circuits is well known for determining the occupancy status of a portion of a railroad by a rail vehicle, such as a train, in order to guarantee safe operation of a railroad system.
Typically, railroad tracks are divided along their length into a plurality of track portions, also called track blocks, each associated with a track circuit. Each track circuit comprises a transmitter unit, which applies a voltage signal between the rails of the railroad track, and a measurement unit, which measures the voltage signal between the rails at a different location. When a train enters the track block, both rails forming the track are electrically connected through the axles of the train, which electrically shunts the track circuit. As a result, the measurement unit detects a variation of the voltage signal, thus indicating the occupancy status of the corresponding track block. Some examples of track circuits are known as DC track circuits or as audio frequency track circuits.
However, the boundary between neighboring track blocks must be clearly defined, in order to avoid any false determination of track block occupancy, which may have adverse consequences. In jointed railroad tracks, this is usually done by using electrically insulated joints between the track sections. In the case of jointless railroad tracks, where no such insulating joints are used, boundaries are usually defined by using impedance bonds that are placed on the track at specific locations. However, the installation of impedance bonds is costly and complicates the maintenance of the track.
Some known audio frequency track circuits purport to solve this problem, by using several receiver units in each track circuit, said receiver units being placed on each side of the transmitter unit, in order to define virtual boundaries. An example of such track circuits is disclosed in U.S. Pat. No. 3,479,502.
However, such known solutions are not entirely satisfactory. One reason is that, due to the extra receiver units, the cabling required to connect the various elements to the tracks is much more complicated. This increased complexity may be detrimental to the installation of the track circuit and complicates its maintenance.